The present invention relates to a magnetic recording medium, which is installed in an external storage device of a computer, and a method for manufacturing such a magnetic recording medium. In particular, the invention relates to an improvement of a magnetic recording medium by means of excellent liquid lubricant applied on the surface of the magnetic recording medium.
A magnetic recording medium is commonly used in a fixed magnetic disc drive as a memory of a computer. In the fixed magnetic disc drive, a CSS (contact start and stop) system is employed where a magnetic head of the disc drive floats when the magnetic recording medium rotates and the magnetic head comes into contact with the surface of the magnetic recording medium when a driving motor which drives to rotate the magnetic recording medium stops.
The magnetic recording medium comprises an underlayer and a magnetic layer formed by dc sputtering on a hard base body. The surface of the magnetic layer is covered with a carbon protective film in order to prevent wear of the magnetic layer and also to reduce frictional force between the magnetic recording medium and the magnetic head. Generally used material for the surface protective layer to prevent friction and wear due to sliding between the magnetic recording medium and the magnetic head includes diamond-like carbon (DLC), amorphous carbon, or DLC with a small amount of additive of N or Si. In the DLC, diamond-like property exhibiting high hardness is raised and a proportion of bonding in diamond structure is larger than that of bonding in graphite structure.
The surface of the protective layer formed of the diamond-like carbon is covered with a thin oxide film having a functional group, such as reactive carbonyl group, carboxyl group, or hydroxyl group, and contaminants actively adsorbs here and bonds to the functional group in the protective layer. Therefore, the surface of the protective layer of the conventional magnetic recording medium is wholly covered with a lubricant, preventing the surface from adsorbing contaminant such as harmful gas or organic substance, and also improving lubrication characteristics, in an attempt to obtain a magnetic recording medium with excellent CSS endurance and stability.
The lubricant layer used for improving lubricating characteristics of the surface of a magnetic recording medium is required to be stably formed with uniform thickness on the surface of the protective film. It is also important that the lubricant exhibit high adhesiveness and bonding strength with the protective film. To increase the adhesiveness, a lubricant has been used which is a perfluoropolyether having a hydroxyl group or piperonyl group at an end or ends of the molecule. Such a lubricant is available in the trade name Z-DOL or AM2001 from Ausimont S.p.A.
The perfluoropolyether lubricant has a poor lubricating characteristic if its molecular weight is too low, and tends to adhere to the magnetic head if the molecular weight is too high. Thus, the perfluoropolyether lubricant having the weight average molecular weight (MW) of 1,500 to 5,500 has been conventionally used. When the molecules with such relatively large molecular weight is applied in the thickness of several tens of Angstroms, gaps between molecules is generated and it becomes difficult for the lubricant layer to wholly cover the surface of the protective layer. Thus, the conventional perfluoropolyether lubricant requires thickness of the lubricant layer of more than 50 xc3x85, which makes it difficult to prevent the magnetic recording medium from adhesion with the magnetic head.
With high-density recording in recent years, the floating height of the magnetic head has been reduced, and the structure of the magnetic head has been changed to employ a low-floating type magnetic head, such as MR head, in place of the heads of conventional type. The material of low-floating head is likely to undergo a catalytic action or generate heat due to friction, which causes decomposition of the perfluoropolyether lubricant in the principal chain portion (ether portion). The decomposed substances or corrosive components of the gas adsorbed on the disc surface are transferred to the surface of the magnetic head, and floating characteristics of the magnetic head is disturbed, causing reduction in the reproduction output. In addition, the decomposed perfluoropolyether lubricant, cannot maintain its lubricating characteristics, causing wear of the protective film, and in the worst case, head crush takes place.
Further, the rotating speed of the magnetic recording medium is increasing from a conventional speed of about 7,200 rpm to a higher speed of 7,200 to 15,000 rpm. As a result, a phenomenon of spin-migration is appearing, that is, the lubricant on the surface of the magnetic recording medium moves or scatters toward a radially outer portion due to a centrifugal force. If the migration becomes excessive, harmful adhesion or, in the worst case, head crash may occur.
As described above, the conventional technologies may deteriorate the performance of a fixed magnetic recording disc drive in a large extent. Although various attempts have been made to use various perfluoropolyether lubricants so as to solve the above-described problems, such a technology that meets all of the above requirements had not been established.
It is therefore an object of the present invention to solve the problems involved in high density recording and high transfer rate wherein decomposition of the lubricant due to low-floating-height of the magnetic head and migration phenomenon due to high-speed rotation arise. It is also an object of the invention to provide a magnetic recording medium that exhibits stable resistance to environmental conditions and stable lubricating performance for a prolonged time. It is another object to provide a method for manufacturing such a magnetic recording medium.
The magnetic recording medium of the invention solves the above-described problems involved in the use of the recording medium together with a magnetic head in a fixed magnetic disc drive. To accomplish the above objects, the present invention provides a magnetic recording medium comprising a base body, a non-magnetic metal underlayer formed on the base body, a magnetic layer formed on the non-magnetic metal underlayer, a protective layer formed on the magnetic layer, and a lubricant layer formed on the protective layer. The lubricant layer of the invention contains a first liquid lubricant and a second liquid lubricant, wherein the first liquid lubricant contains at least one perfluoropolyether having one or two alcohol end groups, selected from a group consisting of perfluoropolyethers represented by general formulas (I), (II) and (III), the general formulas (I), (II) and (III) being expressed by
xe2x80x83R1CH2xe2x80x94(CF2CF2O)lxe2x80x94(CF2O)mxe2x80x94CH2R2xe2x80x83xe2x80x83(I)
F(CF2CF2CF2O)nxe2x80x94CH2R1xe2x80x83xe2x80x83(II)
F(CF2CF2O)nxe2x80x94CH2R1xe2x80x83xe2x80x83(III)
where each of R1 and R2 is independently an end group, each of l and m is 0 or an integer excepting the case l=m=0, and n is an integer, and
the second liquid lubricant contains at least one perfluoropolyether having one or two tertiary amine end groups, selected from a group consisting of perfluoropolyethers represented by general formulas (IV), (V) and (VI), the general formulas (IV), (V) and (VI) being expressed by
R3R4Nxe2x80x94(CH2)pxe2x80x94(CF2CF2O)lxe2x80x94(CF2O)mxe2x80x94(CH2)qxe2x80x94NR5R6xe2x80x83xe2x80x83(IV)
F(CF2CF2CF2O)nxe2x80x94(CH2)pxe2x80x94NR3R4xe2x80x83xe2x80x83(V)
F(CF2CF2O)nxe2x80x94(CH2)pxe2x80x94NR3R4xe2x80x83xe2x80x83(VI)
where each of R3, R4, R5, and R6 is independently an end group excluding a hydrogen atom, each of l, m, p and q is 0 or an integer excepting the case l=m=0, and n is an integer.
Advantageously, the lubricant layer is formed of a mixed liquid lubricant including the first liquid lubricant containing the perfluoropolyether having alcohol end group(s) and the second liquid lubricant containing the perfluoropolyether having tertiary amine end group(s) which are defined above.
Alternatively, the lubricant layer is comprised of a double layer including a first layer formed of the first liquid lubricant containing the perfluoropolyether having alcohol end group(s) and a second layer formed of the second liquid lubricant containing the perfluoropolyether having tertiary amine end group(s).
The perfluoropolyether having tertiary amine end group(s) has preferably a weight average molecular weight of 1,500 to 15,000 and the perfluoropolyether having alcohol end group(s) has preferably a weight average molecular weight of 1,500 to 5,500.
Advantageously, the bonded portion in the lubricant layer which is, strongly bonded with the protective layer is in a proportion of 30 to 100% with respect to a sum of the bonded portion and the mobile portion which is not strongly bonded to the protective layer and mobile relative to the protective layer.
A manufacturing method of a magnetic recording medium in the present invention comprises steps of laminating a non-magnetic metal underlayer on a base body, laminating a magnetic layer on the non-magnetic metal underlayer, laminating a protective layer on the magnetic layer, and laminating a lubricant layer on the protective layer by applying the first liquid lubricant containing the perfluoropolyether having alcohol end group(s) and a second layer formed of the second liquid lubricant containing the perfluoropolyether having tertiary amine end group(s), wherein the first and the second liquid lubricants are defined above.
Advantageously, the step of applying the liquid lubricants is performed by applying a mixed liquid containing the first liquid lubricant and the second liquid lubricant.
Preferably, the step of applying the mixed liquid is performed by applying a mixed liquid by means of dip-coating or spin-coating, the mixed liquid containing the first liquid lubricant and the second liquid lubricant, and being diluted with a solvent.
Alternatively, the step of applying the liquid lubricants is performed in two steps in any order: applying a liquid including the first liquid lubricant and separately applying a liquid including the second liquid lubricant.
Preferably, the two steps include a step of applying a first diluted liquid by means of dip-coating or spin-coating, the first diluted liquid being the first liquid lubricant diluted with a solvent, and a step of applying a second diluted liquid by means of dip-coating or spin-coating, the second diluted liquid being the second liquid lubricant diluted with a solvent.
Advantageously, the method of the invention further comprises a step of heating or ultraviolet ray irradiation such that a bonded portion in the lubricant layer is in a proportion of 30 to 100% with respect to a sum of the bonded portion and a mobile portion.